The general topic of the role of proteins in the processing of RNA has been pursued in several systems. Processing of ribosomal RNA in a BHK ts mutant reveals that some ribosomal structural proteins appear to be missing in nucleolar ribosomal precursor particles under non-permissive conditions for ribosomal synthesis. A second system - rat myoblasts - which cease making ribosomes after differentiation to muscle cells appear to have a severe decrease in the synthesis of ribosomal structural proteins, as well as a possible undermethylation of ribosomal precursor RNA. HnRNA in Hela cell nuclei is associated with specific proteins, one of which appears to bind poly A.